The present invention relates to a compact, lightweight, high-torque permanent magnet type rotary electric machine suitable for use at high temperatures, and also to an electrically driven vehicle using the rotary electric machine.
A driving motor for use in an electrically driven vehicle, especially, in an electric vehicle is desired to have a compact, lightweight configuration and high efficiency, because the capacity of a battery mounted on the electric vehicle is limited and it is necessary to ensure a sufficient distance traveled by the capacity of the battery once fully charged.
To make a motor compact and lightweight, it is desired to be fit for high-speed rotation. Further, as a high-efficient motor, a permanent magnet motor is recommendable rather than a DC motor and an induction motor. In particular, as compared with a surface magnet motor having permanent magnets on the outer circumferential surface of a rotor, a so-called internal magnet motor having a permanent magnet holding portion in a steel plate, e.g., a silicon steel plate, having a permeability higher than that of permanent magnets is suitable for the high-efficient motor. The reason is that the internal magnet motor can be operated up to high speeds by field weakening control and can be operated with high efficiency by field weakening control.
Further, as compared with the rotor of the surface magnet motor, the rotor of the internal magnet motor has an advantage such that the rotational strength of the rotor is determined by the strength of the silicon steel plate, resulting in high reliability in high-speed rotation. An example of such a motor configuration is disclosed in Japanese Patent Laid-open No. 5-76146.
The motor configuration disclosed in this publication is such that permanent magnets are embedded in a rotor core formed of a magnetic material having a permeability higher than that of the permanent magnets, and that auxiliary magnetic poles composed of the permanent magnets and the rotor core are arranged in a circumferential portion of the rotor core. By forming such an internal magnet configuration that the permanent magnets are embedded in the rotor core formed of a magnetic material having a permeability higher than that of the permanent magnets, field weakening control can be performed and the motor can be operated with high efficiency up to a high-speed region.
However, the motor configuration disclosed in the above publication has no consideration on a fixing method for the permanent magnets, especially, on a fixing method for the permanent magnets in the axial direction of the rotor core. Although the above publication describes that the permanent magnets are bonded in holes, there is a possibility that the permanent magnets may axially escape from the holes because of a reduction in adhesive strength by bonding only in the case of a rotary electric machine to be operated at high temperatures.
To cope with this problem, a pair of retainer plates (which will be hereinafter referred to as side rings) for preventing the escape of the permanent magnets are mounted on the axial ends of the rotor. Each side ring is formed of a nonmagnetic material to prevent short of magnetic flux. However, in the case that each side ring is formed of a metal material, an eddy current is generated in each side ring by a change in magnetic flux from stator windings, because of conductivity of the metal material, causing abnormal heating of each side ring. Accordingly, there is a possibility of high-temperature demagnetization of the permanent magnets due to the heat from each side ring.
It is accordingly an object of the present invention to provide a permanent magnet type rotary electric machine which can prevent thermal demagnetization of the permanent magnets to thereby effect a reduction in size and weight and a high torque.
It is another object of the present invention to provide an electrically driven vehicle using the permanent magnet type rotary electric machine.
According to an aspect of the present invention, the outer diameter of each of a pair of retainer plates mounted on the axial ends of a rotor core is set smaller than the outer diameter of the rotor core, thereby suppressing the generation of an eddy current in each retainer plate due to magnetic flux from stator windings.
Preferably, the difference between the outer diameter of the rotor core and the outer diameter of each retainer plate is set to xc2xd or more of the difference between the inner diameter of the stator core and the outer diameter of the rotor core.
According to another aspect of the present invention, each retainer plate is formed of a metal material having a resistivity of 10 xcexcxcexa9cm or higher, thereby suppressing the generation of an eddy current in each retainer plate due to magnetic flux from the stator windings.
According to a further aspect of the present invention, the outer diameter of each retainer plate is set smaller than the outer diameter of the rotor core, and each retainer plate is formed of a metal material having a resistivity of 10 xcexcxcexa9cm or higher, thereby suppressing the generation of an eddy current in each retainer plate due to magnetic flux from the stator windings.
According to a still further aspect of the present invention, each retainer plate is a nonmagnetic member formed of a nonmetal material, thereby suppressing the generation of an eddy current in each retainer plate due to magnetic flux from the stator windings.
According to a still further aspect of the present invention, there is provided an electrically driven vehicle comprising a battery for supplying a DC voltage; an inverter for converting the DC voltage supplied from the battery into an AC voltage; and a permanent magnet type rotary electric machine for outputting a drive torque for driving the vehicle at the AC voltage. The permanent magnet type rotary electric machine in this electrically driven vehicle is the permanent magnet type rotary electric machine according to the present invention.